In conventional modern day motor vehicle passenger compartment heating, ventilating and air conditioning systems, it is common practice to mount a heater core downstream of an evaporator in an air duct and to continuously circulate engine coolant through the heater core. For air conditioning, refrigerant is circulated through the evaporator and temperature control is obtained by controlling the flow of air from the evaporator relative to the heater core. For example, for maximum cooling demand all of the air flow from the evaporator is bypassed around the heater core and thence into the passenger compartment. On the other hand, for minimum cooling demand, all of the air flow from the evaporator is passed through the heater core and thence delivered to the passenger compartment. And intermediate these two extremes, the bypass flow and that through the heater core are mixed and varied to provide an intermediate temperature air delivery to the passenger compartment. Furthermore, the outlets from the duct downstream from the bypass and heater core are strategically placed to provide the best air distribution for the heating, cooling and defrosting modes as is well known. Typically, in such systems, the mode selection is obtained by one air door(s) and the temperature of the air being delivered is controlled by another separate air door(s) upstream of the former. And while such systems have proven very satisfactory, there remains a continuous quest for reduction in both the size of the system and the number of parts making up the system as well as improved air flow, temperature blending and flexible delivery control.